Embodiments of the invention relate generally to magnetic resonance (MR) imaging and, more particularly, to MR pulse sequences for reducing acoustic noise levels during image acquisition.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In existing MR systems, one problem that is encountered is the loud acoustic noise generated by the system. The noise level generated by the MR system can become uncomfortably loud, both for the patient, or subject, and for the operators. The source of such acoustic noise can be many and varied, however, in general, the noise can be attributed to vibration of gradient coils included in the MR system. The noise/vibration from the gradient coils is due to Lorentz forces applied thereto that result from an interaction of a static magnetic field and electrical current, with the Lorentz forces thereby creating vibrations in the gradient coil. Structural borne and airborne noise generated in the gradient coils from the vibrations consequently radiate acoustic noise into the patient bore of the MR system.
There have been attempts at reducing the acoustic noise generated during MR imaging—with some such attempts being focused on reducing the acoustic noise via the pulse sequence employed for acquiring MR imaging data. One such technique is referred to as SWIFT (SWeep Imaging with Fourier Transformation). In SWIFT, time-domain signals are acquired in a time-shared manner during a swept radiofrequency excitation of the nuclear spins—allowing capture of signal from spins with extremely short transverse relaxation time, T2*. The field gradient used for spatial-encoding is not pulsed on and off, but rather is stepped in orientation in an incremental manner, and since the orientation of consecutive projections varies in a smooth manner (i.e., only small increments in the values of the x, y, z gradients occur from view to view), SWIFT scanning is close to inaudible and is insensitive to gradient timing errors and eddy currents. SWIFT, however, is limited regarding T1/PD contrast because of its intrinsic gradient echo property, and thus the image quality obtainable with SWIFT is limited by its intrinsic gradient echo contrasts.
Another MR imaging technique that provides reduced acoustic noise is 3D RADIAL imaging combined with a derated gradient recalled echo (GRE) sequence, which is capable of delivering T1 contrast with an acoustic level only slightly above background noise. 3D RADIAL, however, is not capable of generating clinically usable T2 and FLuid Attenuation Inversion Recovery (FLAIR) contrast. Conversely, while derated 2D fast spin echo (FSE) and 3D FSE (i.e., “3D Cube”) are available techniques that provide T2 and FLAIR contrast, such techniques cannot be considered “silent” applications, as they have an acoustic level of 90 dBA with gradient slew rate derating of 10 T/m/s. While lowering of the acoustic level by further derating the gradient slew rate is possible, such additional derating is not practical, as doing so increases the echo spacing significantly because of large phase encoding gradients that cause image blurriness, SNR loss, phase ghosting artifacts, and motion induced artifacts.
It would therefore be desirable to have a system and method capable of acquiring T2 and FLAIR contrast at reduced acoustic noise levels. It would also be desirable for such acoustic noise level reduction to be achieved for different types of pulse sequences and while minimizing the impact on image quality.